Printed circuit assemblies (PCA's) must be tested after manufacture to verify the continuity of the traces between pads and/or vias on the board and/or to verify that any components loaded on the PCA perform within specification.
Printed circuit assemblies testing requires complex tester resources. The tester hardware must be capable of probing conductive pads, vias and traces on the board under test.
Prior art test fixtures typically employed a bed of nails fixture comprising a large number of nail-like test probes having tips that make electrical contact with the nodal points of the circuit to be tested. The test probes are typically spring loaded pins inserted in receptacles that pass through and are secured relative to a supporting plate (called the probe plate). The printed circuit is placed on top of the test probes and sealed with a gasket. A vacuum is applied through the test fixture to draw the printed circuit board down onto the spring loaded test probes to ensure good electrical contact. The vacuum is maintained until the testing is complete after which another printed circuit board is placed onto the test fixture for testing.
The test probes are inserted into the receptacles which extend below the lower side of the probe plate. The lower end of the receptacle typically has a wire wrap post. A wire is wrapped about the receptacle post and extends in a point-to-point wiring connection to an interface connector pin inserted into a fixture interface panel. The fixture interface panel is adapted to be connected to an interface receiver of the test electronics analyzer. The point-to-point wiring of each receptacle post to a corresponding interface connector pin involves manually wire wrapping wires between each of the receptacle post and interface connector pin.
With the miniaturization of electrical components, the number of test points in a circuit has risen significantly, making point-to-point wiring for each fixture a labor intensive operation. Automation of this process is not economical because each printed circuit board requires a unique design configuration. Furthermore, as the number of test points increases with shrinking technology, the wiring of the closely adjacent test pins becomes more tedious.
Recent advances in fixture technology has led to the use of wireless fixtures. In a wireless fixture, a fixture printed circuit board (PCB) replaces the wires connecting the tester interface pins to the fixture probes with traces on a printed circuit board. In particular, the tester interface pins (either directly or indirectly through tester adapter probes interfacing to the tester pins) make electrical contact with conductive pads on the bottom side of the fixture PCB. The conductive pads on the bottom side of the fixture PCB electrically connect to conductive pads on the top side of the fixture PCB through traces and vias. One end of the probes of the fixture probe plate make electrical contact with the conductive pads on the top side of the PCB, while the other end of the probes of the fixture probe plate make electrical contact with various conductive pads/nodes of the printed circuit board under test. Accordingly, wireless test fixture allows tester pins to make electrical contact with appropriate nodes of the printed circuit board under test without the necessity and complexity of wire-wrap connections.
When a printed circuit board under test is to be tested using a wireless fixture, the tester interface pins (directly or indirectly through tester adapter probes) press on the fixture PCB upward at its bottom conductive pads. Simultaneously, the bottom tips of the fixture probes press against the fixture PCB downward against its top conductive pads. The top tips of the fixture probes press against the bottom conductive pads of the printed circuit board under test.
Although wireless test fixtures have been used for over a decade, the general perception is that the benefits they can theoretically provide are extremely difficult to realize in practice. One of these benefits is signal quality, which consists of whether the right amount of current is delivered and whether it is delivered in its full integrity.
Signal quality is affected by probe and pin contact against the fixture PCB, as well as the width of the PCB traces. Ideally, contact must be maintained with a sufficient force during test, and preserved with a minimum force outside of test to avoid contamination. The traces must be just wide enough to accommodate the delivery of the required current to each device under test. Too thin a trace may result in insufficient current, and too thick a trace may result in PCBs with too many layers, which are difficult and costly to manufacture. Layer count should also be reduced by avoiding unnecessary uses of the fixture PCB internal area; this can be obtained by moving screws to edges and corners, rarely mounting parts on the PCB surface, and minimizing the area occupied by traces and their vias.
There exist several designs of wireless fixtures, differing mainly in the probe types utilized, in mechanical assemblies effecting contact and parallelism, and in PCB design and layout. The main aim is at improving signal quality, fixture cost, PCB layer count, ease of use, and fixture repair.